Capability probe: complex-teaching use case (540 movement)

[a-baran-orhan]

Focused split of Workstream 7 from the dance UAT umbrella #91.

Use the supplied 540 reference as a capability probe, not a promise Posecode can reproduce it.

Tasks

• Break the movement into required semantic events: preparation, support changes, takeoff, free flight, rotation, leg shapes, spotting/focus, landing, recovery.
• Mark which events are expressible with current grammar and which require new semantics.
• Pay particular attention to free flight (currently treated as unsupported in the authoring guide).
• Determine the minimum language/runtime additions needed for a useful teaching visualization.
• Evaluate the claimed advantages: slow playback, scrubbing, alternate camera angles, easy variants.
• Coordinate playback-speed work with Playground: Add UI buttons/slider for animation playback speed control #22 (now shipped — speed control + persistence exist).
• Do not add syntax solely for this example until it generalizes to other complex rotational/airborne movements.

Priority: P2.